1. Field of the Invention
The present invention relates to an imaging tube including an image intensifier, a framing tube, and a streak tube.
2. Description of the Related Art
One conventional imaging tube is an X-ray fluorescence multiplier tube provided with a photocathode and a fluorescent surface, and is disclosed in Japanese Laid-Open Patent Publication SHO-53-67347. The fluorescent surface of this multiplier tube is formed using electrophoretic techniques and is a multi-layer structure consisting of a transparent conductive layer, a fluorescent layer, and a metal thin layer which are sequentially deposited in the stated order on the inner surface of a glass plate (an output faceplate) facing the photocathode.
To improve optical coupling at the output of the imaging tube, a fiber optic plate (FOP) is generally used as an output faceplate. The fluorescent surface of the imaging tube in which the FOP is used is a two-layer structure. Specifically, the fluorescent layer is directly deposited over the inner surface of the FOP and the thin metal layer is deposited over the fluorescent layer. The thin metal layer prevents light generated at the fluorescent layer from feeding back toward the photocathode, and so is called a metal-back film.
Generally, the imaging tubes with FOPs are used in conjunction with a solid-state image pick-up device. In use, the image pick-up device is mounted directly on the FOP. In order to maintain the image pick-up device at ground potential, a transparent conductive layer is formed on the outer surface of the FOP to connect it to ground. On the other hand, because the metal-back thin film is applied with a positive high voltage, a strong electric field is developed between the inner and outer surfaces of the FOP. This strong electric field causes electric charges to appear in the fluorescent layer as a result of leakage currents flowing through the FOP. Due to the electric charges in the fluorescent layer, dark spots are locally observed at the output side of the FOP for a brief period of time when light is uniformly applied to the photocathode. The dark spots finally disappear, because the fluorescent layer which normally has electrical insulation properties exhibits conductive properties when the fluorescent layer generates light, so the electric charges are released from the fluorescent layer soon after the imaging tube is operated.
Further, due to discharges occurring between the metal-back thin film and the FOP caused by the strong electric field developed across the fluorescent layer or by electrons incident into the fluorescent layer from the FOP, bright spots are locally observed at the output side of the FOP when no light is applied to the photocathode. These dark spots and bright spots have the same pattern because these spots are generated resulting from the fact that some fibers of the FOP exhibit conductivity.
While the use of heavily insulated FOPs can prevent the generation of dark and bright spots, that is, degradation of image quality, the expense of heavily insulated FOPs creates an additional problem by increasing the total cost of imaging tubes in which they are used. Also, dark spots and bright spots tend to occur easily even when highly insulated FOPs are used, if the FOPs are slenderized or high voltage is applied thereto.